The present invention relates to a process for polymerizing alpha-olefins.
It is known practice to polymerize alpha-olefins using solid catalysts comprising a transition metal compound containing one or more cyclopentadienyl ligands, an aluminoxane and a support. Patent application EP-A-0 206 794 describes a process for polymerizing ethylene, in which a solid catalyst of this kind is used together with methylaluminoxane or triethylaluminium, which are added to the polymerization medium to capture poisons therein, such as oxygen and water. Such a process leads to the manufacture, in relatively low catalytic yields, of polyethylenes with a low bulk density (BD), which has a negative impact on the production efficiency of the polymerization plants.
Patent application EP-A-314 797 describes a process for polymerizing ethylene using a supported catalyst based on a compound containing cyclopentadiene ligands, an aluminoxane and a dialkylaluminium alkoxide. Such a process leads to the manufacture, in relatively low catalytic yields, of resins containing a large amount of fines.
A polymerization process has now been found which does not have the abovementioned drawbacks and which gives higher catalytic yields of xcex1-olefin polymers with a bulk density which is markedly higher than that obtained in the known processes.
To this end, the present invention relates to a process for polymerizing alpha-olefins, in which at least one alpha-olefin is placed in contact, under polymerizing conditions, with a catalytic system comprising
(a) a solid catalyst comprising (i) a compound of a transition metal from groups 4 to 6 of the Periodic Table, containing at least one cyclopentadiene ligand which may be substituted, (ii) an activator chosen from aluminoxanes and ionizing agents, and (iii) a porous polymeric support with an intraparticulate pore volume, generated by pores with a radius of from 1000 to 75,000 xc3x85, of at least 0.2 cm3/g, and
(b) at least one organoaluminium compound corresponding to the general formula R3xe2x88x92nAl(Yxe2x80x2)n in which 0.9 less than nxe2x89xa63; Yxe2x80x2 represents a group chosen from xe2x80x94ORxe2x80x2, xe2x80x94SRxe2x80x2 and xe2x80x94NRxe2x80x2Rxe2x80x3; R and Rxe2x80x2 independently represent an alkyl group comprising from 1 to 20 carbon atoms, an aryl, alkylaryl or arylalkyl group comprising from 6 to 30 carbon atoms, and Rxe2x80x3 represents a hydrogen atom, an alkyl group comprising from 1 to 20 carbon atoms or an aryl, alkylaryl or arylalkyl group comprising from 6 to 30 carbon atoms.
According to the present invention, the term xe2x80x9calpha-olefinxe2x80x9d means olefins with terminal unsaturation containing from 2 to 20 carbon atoms, preferably from 2 to 8 carbon atoms, such as, more particularly, ethylene, propylene, 1-butene, 1-pentene, 1-hexene and 1-octene.
The solid catalyst (a) used in the process according to the present invention comprises the transition metal compound (i) and the activator (ii) on the support (iii). It goes without saying that compounds other than compounds (i) and (ii) can be supported on the support (iii). Similarly, several compounds (i) and/or (ii) can be supported on the same support.
The transition metal compound (i) which can be used according to the present invention is usually chosen from the compounds of formulae
Qa(C5H5xe2x88x92axe2x88x92bR1b)(C5H5xe2x88x92axe2x88x92cR2c)MeXYxe2x80x83xe2x80x83(1)
and
Qxe2x80x2a(C5H5xe2x88x92axe2x88x92dR3d)ZMeXYxe2x80x83xe2x80x83(2)
in which
Q represents a divalent linking group between the two cyclopentadiene ligands (C5H5xe2x88x92axe2x88x92bR1b) and (C5H5xe2x88x92axe2x88x92cR2c),
Qxe2x80x2 represents a divalent linking group between the cyclopentadiene ligand (C5H5xe2x88x92axe2x88x92dR3d) and the group Z,
a is 0 or 1,
b, c and d are integers which satisfy the conditions 0xe2x89xa6bxe2x89xa65, 0xe2x89xa6cxe2x89xa65 and 0xe2x89xa6dxe2x89xa65 when a is 0 and 0xe2x89xa6bxe2x89xa64, 0xe2x89xa6cxe2x89xa64 and 0xe2x89xa6dxe2x89xa64 when a is 1,
R1, R2 and R3 each represent hydrocarbon-based groups containing from 1 to 20 carbon atoms, which can be linked to the cyclopentadiene ring in the form of a monovalent group, or which can be linked to each other so as to form a ring adjacent to the cyclopentadiene ring, halogen atoms, alkoxy groups containing from 1 to 12 carbon atoms, hydrocarbon-based groups containing silicon, of formula
xe2x80x94Si(R4)(R5)(R6), phosphorus-containing hydrocarbon-based groups of formula xe2x80x94P(R4)(R5), nitrogen-containing hydrocarbon-based groups of formula xe2x80x94N(R4)(R5) or boron-containing hydrocarbon-based groups of formula xe2x80x94B(4)(R5) in which R4, R5 and R6 represent hydrocarbon-based groups containing from I to 24 carbon atoms, provided that when b, c or d is 2 or more and/or when there is a plurality of groups R1, R2 or R3, these groups may be identical or different,
Me represents a transition metal from groups 4 to 6 of the Periodic Table,
Z represents an oxygen atom, a sulphur atom, an alkoxy group or thioalkoxy group containing from 1 to 20 carbon atoms, a nitrogen-containing or phosphorus-containing hydrocarbon-based group containing from 1 to 40 carbon atoms or a hydrocarbon-based group containing from 1 to 20 carbon atoms, provided that one bond of the group Z is linked to the group Qxe2x80x2 when a is 1, and
X and Y, which may be identical or different, each represent a hydrogen atom, a halogen atom, a hydrocarbon-based group, an alkoxy group, an amino group, a phosphorus-containing hydrocarbon-based group or a silicon-containing hydrocarbon-based group containing from 1 to 20 carbon atoms.
The preferred compounds (i) of fon-nula (1) are generally such that
Q represents an alkylene group containing 1 or 2 carbon atoms which can be substituted with alkyl or aryl groups containing from 1 to 10 carbon atoms, or a dialkylgermnanium or dialkylsilicon group containing from 1 to 6 carbon atoms,
a is 0 or 1,
b, c and d are integers which satisfyr the conditions 0xe2x89xa6bxe2x89xa65, 0xe2x89xa6cxe2x89xa65 and 0xe2x89xa6dxe2x89xa65 when a is 0 and 0xe2x89xa6bxe2x89xa64, 0xe2x89xa6cxe2x89xa64 and 0xe2x89xa6dxe2x89xa64 when a is 1,
R1 and R2 represent alkyl, alkenyl, aryl, alkylaryl, alkenylaryl or arylalkyl groups containing from 1 to 20 carbon atoms, several groups R1 and/or several groups R2 possibly being linked to each other so as to form a ring containing from 4 to 8 carbon atoms,
Me is zirconium, hafiiium or titanium,
X and Y represent halogen atoms or hydrocarbon-based groups chosen from alkyls, aryls and alkenyls containing from 1 to 10 carbon atoms.
The compounds which are particularly preferred are those of formula (1) in which Q is a linking group chosen from dimethylsilyl and diphenylsilyl, ethylene and methylenes and ethylenes substituted with alkyl or aryl groups containing from 1 to 8 carbon atoms. Compounds of formula (1) which are particularly suitable are the compounds in which the ligands (C5H5xe2x88x92axe2x88x92bR1b) and (C5H5xe2x88x92axe2x88x92cR2c) are chosen from cyclopentadienyls, indenyls and fluorenyls which may be substituted.
The preferred compounds (i) of formula (2) are usually such that
a is 1,
Qxe2x80x2 represents an alkylene linking group containing 1 or 2 carbon atoms which may be substituted with alkyl or aryl groups containing from 1 to 10 carbon atoms, or a dialkylgermanium or dialkylsilicon group containing from 1 to 6 carbon atoms,
R3 represents an alkyl, alkenyl, aryl, alkylaryl, alkenylaryl or arylalkyl group containing from 1 to 20 carbon atoms, two groups R3 possibly being linked to each other to form a ring containing from 4 to 8 carbon atoms,
Me is zirconium, hafriium or titanium,
X and Y represent halogen atoms or hydrocarbon-based groups chosen from alkyls, aryls, and alkenyls.
Compounds (i) of formula (2) which give good results are the compounds in which the ligand (C5H5xe2x88x92axe2x88x92dR3d) is a cyclopentadienyl, indenyl or fluorenyl which may be substituted and Z is an amino group.
Preferred compounds (i) according to the present invention are the compounds of formula (1).
The activator (ii) is chosen from aluminoxanes and ionizing agents. The term xe2x80x9caluminoxanesxe2x80x9d means compounds corresponding to the formulae R7xe2x80x94(AlR7xe2x80x94O)mxe2x80x94AlR72 and (xe2x80x94AlR7xe2x80x94Oxe2x80x94)m+2 in which m is a number from 1 to 40 and R7 is an alkyl or aryl group containing from 1 to 12 carbon atoms. The preferred compounds are chosen from methyl-, ethyl- and isobutylaluminoxanes and mixtures thereof, and more particularly those in which m is a number from 2 to 20. The compound most particularly preferred is the methylaluminoxane in which m is a number from 10 to 18.
The expression xe2x80x9cionizing agentsxe2x80x9d is intended to denote compounds comprising a first portion which has the properties of a Lewis acid and which is capable of ionizing a transition metal compound (i), and a second portion which is inert with respect to the ionized transition metal compound (i) and which is capable of stabilizing it. Such compounds which may be mentioned are triphenylcarbenium tetrakis(pentafluorophenyl)borate, N,N-dimethylanilinium tetrakis(pentafluorophenyl)borate, tri(n-butyl)ammonium tetrakis(pentafluorophenyl)borate, tris(pentafluorophenyl)boron, triphenylboron, rimethylboron, tris(trimethylsilyl)boron and organoboroxines.
The preferred activators (ii) according to the present invention are aluminoxanes.
The porous polymeric support (iii) can consist of any known polymer which can support the compound (i) and the activator (ii). Non-limiting examples of such polymers which may be mentioned are homo- and copolymers of olefins, of styrenes, of divinylbenzenes and of vinyl chlorides. The support (iii) is preferably a polyolefin support. The term xe2x80x9cpolyolefinsxe2x80x9d means polymers derived from the alpha-olefins defined above or copolymers of these alpha-olefins with each other or with diolefins comprising from 4 to 18 carbon atoms. Preferred supports according to the present invention are homo- or copolymers of ethylene and of propylene.
The polymer particles which can be used as porous polymeric supports (iii) usually have a mean diameter (Ds) of from 5 to 500 xcexcm. Preferably, the mean diameter is greater than or equal to 15 xcexcm and more particularly greater than or equal to 40 xcexcm. Particles with a mean diameter of less than or equal to 200 xcexcm and more particulary less than or equal to 150 xcexcm give good results.
The polymer particles used as porous polymeric support (iii) have an intraparticulate pore volume, generated by pores with a radius of from 1000 to 75,000 xc3x85 (10xe2x88x9210 m) of at least 0.2 cm3/g. In the context of the present invention, the expression xe2x80x9cintraparticulate pore volumexe2x80x9d is intended to denote the pore volume generated, in the region from 1000 to 75,000 xc3x85, by the internal porosity of the particles, excluding the pore volume generated by the porosity between the particles (the interparticulate pore volume). The intraparticulate pore volume generated by pores with a radius of from 1000 to 75,000 xc3x85 can be determined by measuring the total pore volume of the support in the region 1000 to 75,000 xc3x85, followed by subtracting the interparticulate pore volume therefrom. The interparticulate pore volume depends on the mean diameter Ds of the particles (F. Martens and H. Behrens, Plaste und Kautschuk, vol. 20(4), 1973, pages 278-279). The interparticulate pore volume is defined, in the context of the present invention, as being the pore volume generated in the region between   Ds  13
and 75,000 xc3x85.
The support (iii) preferably has an intraparticulate pore volume of at least 0.3 cm3/g. Supports (iii) which are particularly preferred are those with an intraparticulate volume of at least 0.5 cm3/g. The intraparticulate pore volume generally does not exceed 1.5 cm3/g.
The supports (iii) preferably used according to the present invention are those consisting of porous polyolefin particles, and more particularly those with a mean diameter and a pore volume as described above. Such supports are described in particular in patent U.S. Pat. No. 5,556,893 (Solvay). These supports have the advantage of having the desired morphology without them needing to undergo subsequent treatment(s). They have both a very high porosity and very high mechanical abrasion strength, allowing them to be used without them losing their morphology. In addition, the use of supports which are compatible with the polymer leads ultimately to polymers with a particularly low ash content.
The solid catalyst (a) used in the process according to the invention can be obtained by various methods. In general, the support particles (iii) are placed in contact with a solution containing the activator (ii) to give a suspension, which is then evaporated. The solution containing the activator (ii) is generally prepared from liquid aliphatic or cycloaliphatic hydrocarbons which are possibly halogenated, or from liquid aromatic hydrocarbons. Preferred examples of these solvents which may be mentioned are benzene, toluene, xylene, hexane, heptane, octane, decalin, dichloromethane, dichloroethane, chloropropane and chlorobenzene. The transition metal compound (i) can be introduced into the suspension described above. It may also have been incorporated into the support (iii) before it is used. Finally, it can be placed in contact with the support particles comprising the activator (ii). The preferred method for preparing the solid catalyst (a) comprises the preparation of a solution containing the transition metal compound (i) and the activator (ii), to which is added the support (iii) so as to form a suspension, which is then evaporated.
The solid catalyst (a) used in the process according to the present invention generally contains from 0.0001 to 0.5 g of transition metal compound (i) per gram of support (iii). Preferably, the concentration of compound (i) is at least 0.0005 g and more particularly at least 0.001 g per gram of support (iii). Amounts of compound (i) of less than or equal to 0.3 g and preferably less than or equal to 0.1 g per gram of support give good results.
The amount of activator (ii) in the solid catalyst depends on the type of activator used. When the activator (ii) is an aluminoxane, the amount of aluminoxane is usually such that the atomic ratio between the aluminium of the aluminoxane and the transition metal of the compound (i) in the solid catalyst is from 20 to 5000. Preferably, this ratio is at least 50, more particularly at least 100. Good results are obtained when this ratio is at least 200. Usually, the aluminoxane is used in amounts such that the aluminium/transition metal atomic ratio is not more than 2000 and more particularly not more than 1500. Ratios of not more than 1000 give good results. When the activator (ii) is an ionizing agent, the amount of ionizing agent is usually such that the molar ratio between the ionizing agent and the transition metal compound (i) is from 0.05 to 50. Preferably, this ratio is at least 0.1 and more particularly not more than 20.
According to one advantageous variant of the process according to the invention, a solid catalyst (a) is used which has been subjected to a preliminary polymerization in the course of which it is placed in contact with an alpha-olefin, under polymerizing conditions so as to form from 0.01 to 50 g of polyolefin per g of solid catalyst containing the compounds (i), (ii) and (iii). The alpha-olefin used during the preliminary polymerization step is advantageously chosen from alpha-olefins containing from 2 to 4 carbon atoms. Ethylene and propylene are particularly suitable. The amount of polymer formed during the preliminary polymerization step is usually at least 0.05 and more particularly at least 0.1 g of polyolefin per g of solid catalyst containing the compounds (i), (ii) and (iii). Good results are obtained when this amount is less than or equal to 30 g, preferably not more than 10 g, per g of solid catalyst containing the compounds (i), (ii) and (iii). According to one particularly advantageous embodiment of the A invention, this preliminary polymerization is carried out in a diluent whose kinematic viscosity, measured at 20xc2x0 C., is from 3 to 3000 mm2/s (preferably from 10 to 500 mm2/s), such as a mineral oil.
The solid catalyst (a) used in the process according to the invention is generally in the form of a free-flowing dry powder. The particles of solid catalyst generally have the same morphology as the supports from which they are derived. The solid catalyst (a) can be used without further processing for the polymerization of the alpha-olefins. The solid catalyst (a) can also be used in the process according to the invention in the form of a suspension in a diluent which is suited to its use.
The organoaluminium compound (b) used in the process according to the present invention is chosen from compounds corresponding to the general formula R3xe2x88x92nAl(Yxe2x80x2)n in which 0.9 less than nxe2x89xa63, Yxe2x80x2 represents a group chosen from xe2x80x94ORxe2x80x2, xe2x80x94SRxe2x80x2 and xe2x80x94NRxe2x80x2Rxe2x80x3; R and Rxe2x80x2 independently represent an alkyl group comprising from 1 to 20 carbon atoms or an aryl, alkylaryl or arylalkyl group comprising from 6 to 30 carbon atoms, and Rxe2x80x3 represents a hydrogen atom, an alkyl group comprising from 1 to 20 carbon atoms or an aryl, alkylaryl or arylalkyl group comprising from 6 to 30 carbon atoms.
The preferred organoaluminium compounds (b) are those in which R and Rxe2x80x2 independently represent an alkyl group comprising from 1 to 6 carbon atoms and Rxe2x80x3 is a hydrogen atom or an alkyl group comprising from 1 to 6 carbon atoms.
Preferably, the organoaluminium compound (b) used is chosen from compounds corresponding to the general formula R3xe2x88x92nAl(ORxe2x80x2)n in which R and Rxe2x80x2 have the meanings given above. Compounds which are particularly preferred are those corresponding to the general formula R3xe2x88x92nAl(ORxe2x80x2)n in which R and Rxe2x80x2 are alkyl groups comprising from 1 to 6 carbon atoms, and more especially those in which R and Rxe2x80x2 are alkyl groups comprising from 2 to 4 carbon atoms.
The preferred organoaluminium compounds (b) are those in which 1.0 less than nxe2x89xa62.9, and more particularly those in which 1.05 less than nxe2x89xa62.5.
The organoaluminium compounds (b) which are particularly preferred in the process according to the invention are compounds corresponding to the general formula R3xe2x88x92nAl(ORxe2x80x2)n in which 1.05 less than nxe2x89xa62.5 and R and Rxe2x80x2, independently, are chosen from ethyl, isopropyl, isobutyl, n-butyl and t-butyl groups.
The organoaluminium compounds (b) used in the process according to the invention can be obtained by various known methods. They can be obtained, for example, by reacting, in suitable amounts, an organoaluminium derivative of formula R3Al with an alcohol of formula Hxe2x80x94Oxe2x80x94Rxe2x80x2, an amine of formula Hxe2x80x94NRxe2x80x2Rxe2x80x3 and/or a thioalcohol of formula Hxe2x80x94Sxe2x80x94Rxe2x80x2. The organoaluminium compounds (b) used in the process according to the invention, in which n greater than 1, are advantageously obtained by reacting a compound of formula R2Al(Yxe2x80x2) with an alcohol of formula Hxe2x80x94Oxe2x80x94Rxe2x80x2, an amine of formula Hxe2x80x94NRxe2x80x2Rxe2x80x3 and/or a thioalcohol of formula Hxe2x80x94Sxe2x80x94Rxe2x80x2; in these formulae, Rxe2x80x2 and Rxe2x80x3 have the meanings given above in relation to the organoaluminium compound (b). In this case, the amount of alcohol, amine or thioalcohol used is generally less than 2.5 mol per mole of compound of formula R2Al(Yxe2x80x2). Preferably, the amount of alcohol, amine or thioalcohol used is less than or equal to 2 mol per mole of compound of formula R2Al(Yxe2x80x2). An amount which is particularly preferred is one which does not exceed 1.5 mol per mole of compound of formula R2Al(Yxe2x80x2). The amount of alcohol, amine or thioalcohol used is preferably at least equal to 0.05 mol per mole of compound of formula R2Al(Yxe2x80x2).
The organoaluminium compound (b) corresponding to the general formula R3xe2x88x92nAl(Yxe2x80x2)n in which 0.9 less than nxe2x89xa63 can also be obtained by mixing together several organoaluminium compounds, this mixture having a composition such that it corresponds to the general formula R3xe2x88x92nAl(Yxe2x80x2)n in which 0.9 less than nxe2x89xa63. For example, mixing one equivalent of an organoaluminium compound R3Al with one equivalent of a compound R3xe2x88x92xAl(Yxe2x80x2)x in which x greater than 1.8 produces an organoaluminium compound corresponding to the general formula R3xe2x88x92nAl(Yxe2x80x2)n in which 0.9 less than n less than 3.
The organoaluminium compound (b) corresponding to the general formula R3xe2x88x92nAl(Yxe2x80x2)n in which 0.9 less than nxe2x89xa63 can be in various forms, in particular in monomeric, dimeric, trimeric, tetrameric or oligomeric form.
The amount of organoaluminium compound (b) corresponding to the general formula R3xe2x88x92nAl(Yxe2x80x2)n in which 0.9 less than nxe2x89xa63 used in the process according to the invention is generally such that the atomic ratio between the aluminium from the organoaluminium compound (b) and the transition metal of the compound (i) is from 10 to 50,000. Preferably, this ratio is at least 50, more particularly at least 100. Good results are obtained when this ratio is at least 200. Usually, the organoaluminium compound (b) is used in amounts such that the atomic ratio:aluminium from the organoaluminium compound/transition metal of compound (i) is not more than 20,000 and more particularly not more than 17,000. Ratios of not more than 15,000 give good results.
The polymerization process according to the invention can be carried out in continuous or batchwise mode, according to any known process, in solution or in suspension in a hydrocarbon-based diluent, in suspension in the monomer, or one of the monomers, maintained in the liquid state or in the gas phase.
The temperature at which the polymerization process according to the invention is carried out is generally from xe2x88x9220xc2x0 C. to +150xc2x0 C., usually from 20 to 130xc2x0 C. The polymerization temperature is preferably at least 60xc2x0 C. Preferably, it does not exceed 115xc2x0 C.
The total pressure at which the process according to the invention is carried out is generally chosen between atmospheric pressure and 100xc3x97105 Pa, more particularly between 10xc3x97105 and 55xc3x97105 Pa.
The molecular mass of the polymers manufactured according to the process of the invention can be adjusted by adding one or more agents for adjusting the molecular mass of the polyolefins, such as, more particularly, hydrogen.
According to one advantageous variant of the process according to the invention, the process comprises a first polymerization step, which is separate from the preliminary polymerization step (described above in relation to the solid catalyst) and referred to as the prepolymerization step, during which from 1 to 1000 g of polymer is formed per g of solid catalyst containing the compounds (i), (ii) and (iii). The amount of prepolymer formed in this prepolymerization step is advantageously at least 3 g, more particularly at least 5 g, per g of solid catalyst containing the compounds (i), (ii) and (iii). Good results are obtained when the amount of prepolymer is not more than 700 g, more particularly not more than 400 g, per g of solid catalyst containing the compounds (i), (ii) and (iii). In general, the prepolymerization step is carried out at a temperature of from 0 to 60xc2x0 C., preferably at a temperature of from 20 to 50xc2x0 C.
When the polymerization process is applied to the polymerization of propylene, this prepolymerization step is advantageously carried out in suspension in the liquid monomer. When the process is applied to the polymerization of ethylene, the prepolymerization is advantageously carried out in a diluent chosen from aliphatic hydrocarbons containing from 3 to 10 carbon atoms.
One advantage of processes comprising such a prepolymerization step is that the morphology of the polymer is conserved even when the subsequent polymerization step is carried out at high temperature.
The polymerization process according to the invention is advantageously applied to the manufacture of ethylene polymers, and more particularly to the manufacture of ethylene homopolymers and copolymers comprising at least 90mol% of units derived from ethylene. The preferred copolymers are those of ethylene and of another alpha-olefin comprising from 3 to 8 carbon atoms. Copolymers of ethylene and of 1-butene and/or of 1-hexene are particularly preferred. In this case, the polymerization process is preferably carried out in suspension in a hydrocarbon-based diluent. The hydrocarbon-based diluent is generally chosen from aliphatic hydrocarbons containing from 3 to 10 carbon atoms. Preferably, the diluent is chosen from propane, isobutane and hexane or mixtures thereof.
The process according to the invention is also advantageously applied to the manufacture of copolymers of ethylene and of another alpha-olefin comprising from 3 to 8 carbon atoms, with a bimodal molecular weight distribution. According to a first variant of the process according to the invention, these copolymers are manufactured in a single polymerization reactor using a catalytic system comprising at least two different transition metal compounds (i), each giving a polymer whose molecular mass is different from that of the other. According to a second variant of the process according to the invention, these polymers are manufactured by carrying out the process according to the invention in at least two polymerization reactors connected in series, the polymerization conditions being different in the two reactors. In this second variant, the polymerization process is preferably carried out so as to obtain an ethylene copolymer of high molecular mass in one of the reactors, and an ethylene homopolymer whose molecular mass is lower than that of the copolymer, in the other reactor.
The process according to the invention produces alpha-olefm polymers with a high bulk density (BD), in particular markedly higher than the polymers obtained in processes using a catalytic system not comprising an organoaluminium compound (b) according to the invention. The reason for this is that it has been observed, surprisingly, that the use of an organoaluminium compound (b) corresponding to the formula R3xe2x88x92nAl(Yxe2x80x2)n gives polymers whose bulk densities are higher than those obtained with the same solid catalyst but in the absence of an organoaluminium compound or in the presence of a conventional alkylaluminium such as trimethylaluminium, triethylaluminium or triisobutylaluminium. The advantage of obtaining polymers with high bulk densities is that it increases the production capacities of the polymerization plants, and the storage and transportation capacities.
In addition, it has also been demonstrated, surprisingly, that the process according to the invention gives very high catalytic activity, much higher than that obtained when a non-porous support or an inorganic support such as silica is used as support (iii) and/or when the catalytic system is used without an organoaluminium compound (b) or in the presence of a conventional alkylaluminium such as trimethylaluminium, triethylaluminium or triisobutylaluminium.
Another advantage of the process according to the invention is that there is virtually no formation of crust in the polymerization reactor.
Moreover, the use of the process according to the invention produces polymers with very good morphology and whose content of fine particles (particles with a diameter of less than or equal to 125 xcexcm) is very low, usually less than 0.5% by weight and more particularly less than 0.1% by weight relative to the total weight of polymer. Such low fines contents are obtained even for high catalytic activities (less than 0.5 ppm of transition metal in the final polymer).
The examples which follow serve to illustrate the invention. The meaning of the symbols used in these examples, the units expresssing the magnitudes mentioned and the methods for measuring these magnitudes are explained below.
Ds=mean diameter of the support particles, in Itm. The mean diameter of the support particles is the median diameter of the particles measured, using a suspension in 2-propanol, according to standard NFX11-666 (1984) on a Malvem(copyright) Mastersizer MS1000 machine.
PVi=intraparticulate pore volume of the support, generated by pores with a radius of from 1000 to 75,000 xc3x85 (10xe2x88x9210 m), expressed in cm3/g. The porosity of the supports (iii) is determined by the mercury penetration method using porosimeters sold by Carlo Erba Co. in the pore radius region 75 to 75,000 xc3x85 (10xe2x88x9210 m). This gives the curve of the total pore volume expressed in cm3/g as a function of the pore diameter, from which the total pore volume generated by pores with radii of from 1000 to 75,000 xc3x85 (10xe2x88x9210 m) is determined. The intraparticulate pore volume is obtained by subtracting the interparticulate pore volume (which is the pore volume generated by pores with a radii of between   Ds  13
(value expressed in xc3x85) and 75,000 xc3x85) from this total pore volume.
xcex1=catalytic activity, expressed in kg of polymer obtained per millimole of transition metal from the compound (i). This catalytic activity is assessed indirectly from the determination of the residual content of transition metal in the polyethylene by Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) on a Micromass(copyright) Plasma Trace 1 machine.
BD=bulk density of the polymer obtained, expressed in kg/m3. The bulk density (BD) of the alpha-olefin polymer is measured by freeflow according to the following procedure: the polymer from the polymerization process is poured into a cylindrical 50 cm3 container, taking care not to pack it down, from a hopper whose bottom edge is 20 mm above the top edge of the container. The container filled with the powder is then weighed, the tare is deducted from the weight recorded and the result obtained, expressed in kg, is multiplied by 20,000, so as to express the BD in kg/m3.